Heterogeneous Tandem Catalysis Strategy for Additive-Free CO₂ Hydrogenation into Formic Acid in Water: Crystal Plane Effect of Co₃O₄ Cocatalyst

The transformation of carbon dioxide (CO2) into formic acid (FA; HCOOH) in an aqueous phase is a promising method of realizing an environmentally friendly FA/CO2-mediated chemical hydrogen storage/supply cycle. Despite progress in the design of catalysts that operate under basic conditions, the development of efficient catalysts that operate under additive-free conditions lags behind owing to the difficulty in activating CO2 and the low solubility of CO2 in pure water. In the present study, we present a heterogeneous tandem catalysis strategy in which Co3O4 is used as a CO2 hydration cocatalyst to produce a HCO3– intermediate, in combination with our previously reported PdAg/TiO2 as a catalyst for the hydrogenation of HCO3– to afford FA. The turnover number based on Pd improved by a factor of more than 8 in the presence of the Co3O4 cocatalyst with a cubic particle morphology enclosed by (100) facets. A series of morphology-controlled Co3O4 cocatalysts was investigated to elucidate the effect of the exposed crystal facets (i.e., (100), (111), or (112)) on their physicochemical properties and catalytic activity in FA synthesis. A systematic comparison based on experimental and density functional theory calculations demonstrated that the substantial enhancement effect of the Co3O4 cubes is attributable to the in situ generation of the largest amount of surface Co–OH groups with strong basicity originating from the exposed (100) facets. In addition, the present tandem catalytic system displayed high recyclability without exhibiting a structural change or a significant loss of activity. These findings will allow the rational design of an environmentally benign catalytic system for the hydrogenation of CO2 to FA.